Java If…Else

Introduction to Java If…Else

In Java, the if. . .else statement is used to execute specific blocks of code based on a condition. This control flow mechanism is essential for making decisions in a program, allowing it to choose different paths of execution depending on the evaluation of boolean expressions.

Syntax of If…Else

If Statement: Executes a block of code if the condition is true.
Syntax:

if (condition) {
    // Code to execute if condition is true
}

If-Else Statement: Provides an alternate block of code to execute if the condition is false.
Syntax:

if (condition) {
    // Code to execute if condition is true
} else {
    // Code to execute if condition is false
}

If…Else If…Else Statement: Allows checking multiple conditions sequentially.
Syntax:

if (condition1) {
    // Code to execute if condition1 is true
} else if (condition2) {
    // Code to execute if condition2 is true
} else {
    // Code to execute if none of the above conditions are true
}

How If…Else Works

1. Condition Evaluation:
   • The condition inside the if statement is evaluated.
   • If the condition evaluates to true, the corresponding block of code executes.
   • If it evaluates to false and an else statement is present, the else block executes.
2. Sequential Checks:
   • For multiple conditions in if…else if…else, Java evaluates each condition sequentially until it finds one that is true.
   • If none of the conditions are true, the else block executes as a default case.

Examples of If…Else

Basic If Statement

public class IfExample {
    public static void main(String[] args) {
        int age = 20;

        if (age >= 18) {
            System.out.println("You are eligible to vote.");
        }
    }
}

Output:

You are eligible to vote.

If…Else Statement

public class IfElseExample {
    public static void main(String[] args) {
        int marks = 45;

        if (marks >= 50) {
            System.out.println("You passed the exam.");
        } else {
            System.out.println("You failed the exam.");
        }
    }
}

Output:

You failed the exam.

If…Else If…Else Statement

public class IfElseIfExample {
    public static void main(String[] args) {
        int marks = 85;

        if (marks >= 90) {
            System.out.println("Grade: A+");
        } else if (marks >= 75) {
            System.out.println("Grade: A");
        } else if (marks >= 50) {
            System.out.println("Grade: B");
        } else {
            System.out.println("Grade: F");
        }
    }
}

Output:

Grade: A

Nested If…Else

An if…else statement can also be nested inside another if…else to handle more complex conditions.

Example:

public class NestedIfExample {
    public static void main(String[] args) {
        int age = 25;
        boolean hasVoterID = true;

        if (age >= 18) {
            if (hasVoterID) {
                System.out.println("You can vote.");
            } else {
                System.out.println("You need a voter ID to vote.");
            }
        } else {
            System.out.println("You are not eligible to vote.");
        }
    }
}

Output:

You can vote.

Use Cases of If…Else

1. Validation: Checking user input or program states.
2. Decision Making: Executing different blocks of code based on conditions.
3. Handling Multiple Scenarios: Using else if to handle various cases.

Common Mistakes in If…Else

Missing Braces:
Omitting braces { } can lead to logical errors, especially when adding new statements.
Incorrect:

if (a > b)
    System.out.println("A is greater.");
    System.out.println("This will always execute.");

Assignment Instead of Comparison:
Using = instead of == inside the condition.
Incorrect:

if (a = b) { // Assignment instead of comparison
}

Unreachable Code:
Placing code after a return statement in an if block.

Best Practices for If…Else

Keep Conditions Simple: Avoid writing overly complex conditions.
Example:

if ((a > b && c < d) || e == f) {
    // Clear and easy to understand
}

Use Meaningful Variable Names: Make conditions self-explanatory.
Example:

if (isUserLoggedIn) {
    System.out.println("Welcome!");
}

Avoid Deep Nesting: Refactor deeply nested if. . .else for better readability.
Example:

if (condition1 && condition2) {
    // Execute code
}